The present invention relates to a cathode ray tube, and particularly to a cathode ray tube wherein a plurality of electrode layers forming a main lens along with electrodes of the electron gun are provided on the inner wall of the neck.
Generally, a cathode ray tube is constructed such that a phosphor layer constituting a screen is formed on the inside flat bottom of an envelope, an electron gun is inserted into the neck opposing the flat bottom of the envelope, a conductive layer is formed on the inner surface of the envelope's conical portion between the neck and the screen, and a deflection yoke is installed around the neck adjacent to the exiting region of the electron beam being emitted from the electron gun. While the funnel and panel of the envelope are coupled in color cathode ray tubes, the funnel and panel of most monochrome cathode ray tubes are united as one body without being separated.
The electron gun of such a conventional cathode ray tube comprises a triode consisting of a cathode, a control electrode, and a screen electrode, and a main lens consisting of a focusing electrode and an anode. To weaken the influence of spherical aberration and astigmatism on the electron beam emitted from the electron gun, the main lens is composed of at least three electrodes, thereby controlling the electron beam by multiple steps. By this operation, however, the electron gun is considerably lengthened, which in turn lengthens the neck housing the electron gun, i.e., the total length of cathode ray tube.
In order to minimize the influence of the electron lenses' spherical aberration of the electron beam, the diameter of the lenses' apertures must be enlarged. In an effort to solve the problem, a technique has been suggested in U.S. Pat. No. 2,726,348 as illustrated in FIG. 1. Here, an accelerating electrode layer 15 is electrically connected to an inner graphite layer 1b on the conical portion, and serves as the last accelerating electrode which is provided on the inner surface of neck 1a adjacent to the electron beam emitting region of electron gun 1' of cathode ray tube 1. Here, accelerating electrode layer 15 functions as the last accelerating electrode, and surrounds only a portion of the end of focusing electrode 14 which is the last electrode, and not cathode 11, control electrode 12, or screen electrode 13.
In this conventional cathode ray tube, the spherical aberration can be improved. However, it is disadvantageous in that the neck of the cathode ray tube must be considerably lengthened to improve the focusing characteristic of electron beam.